1. Field of the Invention
The present invention relates to liquid crystal cell image projection systems. More specifically, the present invention relates to the use of optical energy to produce a uniform texture of scattering centers in a liquid crystal cell and an associated optical darkening of an image produced by this cell.
2. Summary of the Prior Art
Liquid crystal cells are useful in display, optical processing, printing and related image projection applications. In these applications they serve as light valves or light modulators which control the flow of light from a light source to a receiver or receiving surface. When suitable spatial electronic addressing means is incorporated in a liquid crystal light valve system, spatially varying patterns can be written on the liquid crystal layer. The liquid crystal cell then becomes a spatial light modulator. When suitable illumination optics, typically including a light source and optical condenser, and suitable projecting optics, typically including a projection lens with appropriate aperture, are included in a liquid crystal light valve system, the liquid crystal cell then becomes an electronic slide which determines and controls the image projected onto a display screen, photosensitive detector array or photosensitive writing medium for display, optical processing, and printing applications, respectively.
The spatial variations of the image on the liquid crystal cell virtually always correspond to differences in the molecular orientation (different textures) of the liquid crystal in different regions of the liquid crystal layer. The spatially varying differences in molecular orientation are converted into spatially varying light intensity variations by means of suitable polarizing optics or suitable apertures to selectively pass and block scattered or refracted light from these spatially different regions. The various possibilities have been reviewed in detail (see F. J. Kahn, "The Molecular Physics of Liquid Crystal Devices," Physics Today, May 1982).
In one mode of operation the entire image area of the liquid crystal cell is switched, typically by electrical or thermal means, into a texture with uniform order (or disorder) thereby providing a uniform image background. One or more spatially varying images with different textures are then superposed, by the spatial addressing means, onto the uniform image background in order to create an image on the liquid crystal cell, and hence on the receiving surface, with the desired spatial variations in intensity and contrast.
This invention relates to improved methods and apparatus for creating uniform image backgrounds. This is an area of generic importance not only for display and imaging applications as described above, but is also of importance for other liquid crystal applications such as nondestructive thermal testing using temperature sensitive liquid crystals.
Referring to FIG. 1, a cross sectional view of a typical prior art liquid crystal cell 10 is shown. The cell 10 is filled with liquid crystal 20. A suitable liquid crystal is a smectic A liquid crystal. The crystal is contained on two sides by a first and second electrode 16 and 18. The electrodes 16 and 18 are substantially parallel plates made of transparent material. A suitable material for the electrodes is indium-tin-oxide. The electrodes 16 and 18 serve to allow application of an electric field in a direction essentially normal to the liquid crystal layer while allowing projection light to be transmitted through the liquid crystal layer. The liquid crystal cell 10 is further comprised of a first and second transparent substrate, 12 and 14, typically glass. Among other things, the transparent substrates permit the passage of light, support the electrodes, and provide integrity to the liquid crystal cell.
In the image projection system context, liquid crystal cell image projection systems incorporate means for creating an image on a liquid crystal cell and then use the created images as a mask for projection exposure of photosensitive media for photolithographic processing such as fabrication of printed wiring boards. One of the initial steps in producing an image on a liquid crystal cell involves forming a uniform texture of scattering centers. Scattering centers refer to the characteristic texture of the liquid crystal material in the cell 10. Scattering centers scatter and depolarize light. Conversely, liquid crystal regions without scattering centers transmit light without deflecting it or depolarizing it. A uniform texture of scattering centers is a macroscopic volume of liquid crystal cell 10 which scatters incident light.
The uniform texture of scattering centers is then "drawn" on by a laser beam to create an image. The laser beam etches an image in the uniform texture of scattering centers by creating non-scattering regions wherever it impinges. The process of impinging laser light on the uniform texture of scattering centers to produce nonscattering centers is known in the art, (F. J. Kahn, "Locally Erasable Thermo-Optic Smectic Liquid Crystal Storage Displays," U.S. Pat. No. 3,796,999 dated Mar. 2, 1974).
The prior art contains two methods of rapidly producing large area uniform scattering textures by methods other than laser-scanning which is relatively slow. Both of these methods involve the use of electrical energy. In the first an electrical current pulse through the electrode is used to generate heat in the liquid crystal, the electrode heating method. In the second an electrical current pulse through the liquid crystal is used to create a turbulent material flow in the liquid crystal, the current-turbulence method. In the first method (see U.S. Pat. No. 4,799,770 to Frederic J. Kahn for a Liquid Crystal Cell for Image Projection and Method of Operating Same, issued Jan. 24, 1989), a signal from a voltage source 22 is applied across one of the electrodes. This method is illustrated in FIG. 1 where the voltage signal from the source 22 is applied across the electrode 18. In this embodiment, short electrical pulses are used to thermally induce a uniform texture of scattering centers over the substantially large surface area of the cell 10. The result, in an image projection system, is that the short electrical pulses result in a fast darkening of the image projected on the receiving surface. The electrical current sent through the electrode 18 produces thermal energy which raises the temperature of the liquid crystal material 20. Also, because it is a short pulse, the energy does not have time to spread deeply into the substrate. This lack of spreading facilitates rapid cooling which is responsible for the creation of a high density scattering centers. Physically, the liquid crystal is heated to a temperature at which it is an isotropic state. It then cools rapidly, quenching in some of the disorder of the isotropic state, and thereby produces a high density of scattering centers.
In an alternate embodiment of the prior art, the electrical pulses are applied across the liquid crystal 20. In this embodiment, following the work of Tani (C. Tani, Applied Physics Letters 19, 241-2, Oct. 1, 1973. "Novel Electro-Optical Storage Effect in a Certain Smectic Liquid Crystal"), a dc electric field applied across a thin layer of smectic A liquid crystal causes a current to flow in the liquid crystal. The current produces a turbulent flow of the liquid crystal analogous to the well-known dynamic scattering effect in nematic liquid crystals. This turbulence can produce a high density of optical scattering centers in smectic liquid crystal cells with appropriate conductivity and appropriate orienting layers on the substrates, and can also be produced by low frequency ac voltages as well as by dc voltages. See for example, W. A. Crossland and S. Canter, "An Electrically Addressed Smectic Storage Device," SID '85 Digest, pp. 124-126 (1985) and W. A. Crossland, J. H. Morrissy, and B. Needham, "Method for Preparing and Operating a Smectic Liquid Crystal Display Cell Having Indefinite Storage Properties," U.S. Pat. No. 4,139,273 (Feb. 13, 1979).
One problem with the first method is that small cosmetic defects in the electrodes, e.g., pinholes, etc., are not conductive and, therefore, do not produce the requisite heat to create scattering centers. For example, when a pinhole occurs in an electrode, the heat generated by the electrical current pulse is reduced not only in the area of the pinhole, but also in an area surrounding the pinhole. This phenomena is even more dramatic with a scratch which is oriented so as to divert the flow of current. Although the scratch may be barely visible on the face of the conducting electrode, not only the area of the scratch, but the surrounding area of the scratch is depleted of current and, therefore, of thermally generated scattering centers. In some typical applications such as display (U.S. Pat. No. 4,799,770 and Fabrication of Printed Wiring Boards, pending application U.S. Ser. No. 262,471), electrode defects as small as a few microns or less will be visible on the final projected images or affect the quality or positioning of image features such as line edges. Line edge positioning is particularly critical in the Printed Wiring Board Application where a typical line edge position tolerance of .+-.10% for a 0.003" wide wiring trace will correspond to a position tolerance of .+-.1.25 micrometer for the line edge on the liquid crystal cell at 6.times. projection magnification.
Another problem with electrically induced darkening is the deleterious effects of the required high voltage and current on the liquid crystal cell 10. The chemical and physical properties of the cell 10 may breakdown under the stress of high voltage and current surges associated with electrical darkening. Additionally, the high voltages and high current necessary to produce the desired short electric pulses require a relatively substantial amount of power.
Furthermore, considerable fabrication process work and control is required in order to tailor the electrical and geometric properties of the electrodes and liquid crystal to achieve the desired density and uniformity of scattering centers. For example, in the electrode heating method, the electrode must typically have a very low resistivity per unit area in order to minimize voltage across the liquid crystal which could result in electrical breakdown. As a result, contacts or busbars with much lower resistivity per unit area than the electrodes must typically be provided. The busbars minimize the voltage drop along the length of each contact thereby resulting in a spatially uniform current flow through the electrodes. (See F. J. Kahn et al, "A Paperless Plotter Display System Using a Laser Smectic Liquid-Crystal Light Valve, " SID 87 Digest, pp. 254-257). The busbars must be patterned, thus patterning steps are required in the fabrication process. These steps can result in cosmetic defects, increase process cost and reduce yield.
In the current-turbulence method, the liquid crystal typically must have conducting additives or dopants in it, in order to allow enough current flow to create the scattering center inducing turbulence. But the passage of current through the liquid crystal typically results in electrochemical processes which cause the devices to fail. Thus other dopants, such as redox dopants, must be added to the liquid crystal to counteract these electrochemical processes and extend lifetime. Finally processes need be developed to distribute these dopants uniformly through the liquid crystal cell and special low resistance contacts or busbars must be provided (requiring expensive and yield reducing patterning steps in the fabrication process). Thus, implementing either of these prior art methods results in considerable additional expense in fabrication of the liquid crystal cells.